In recent years, non-aqueous electrolyte secondary batteries have been developed, which are capable of fast charge and high power discharge as well as have an excellent cyclic performance. Such non-aqueous electrolyte secondary batteries are thus suitable as vehicle secondary batteries to be mounted on hybrid vehicles and electric vehicles and as power storage secondary batteries used for electric power leveling.
The secondary battery as described above has a structure in which an electrode group and an electrolyte solution impregnated in the electrode group are housed in a battery container as described in Japanese Patent Application Publication No. 2009-26490. In the structure, the electrode group is formed by winding up a strip-shaped positive electrode and a strip-shaped negative electrode with a separator interposed therebetween, and forming the resultant body in a flat shape.
Moreover, at both front and back end portions of the electrode group, metal foils serving as a current collector of the positive electrode and metal foils serving as a current collector of the negative electrode protrude out while being overlapped with one another by the winding up, thereby forming current collecting tabs 2a, 2b as illustrated in FIG. 1. A lead 5, which electrically communicates with an external terminal for taking out a current, is connected to the current collecting tabs 2a, 2b. 
The lead 5 is connected to the current collecting tabs 2a, 2b by ultrasonic bonding in a state where the lead 5 is overlapped with the metal foils of the current collecting tabs 2a, 2b, which protrude out at each of the front and back end portions of a power generating element. However, if the lead 5 and the metal foils of the current collecting tabs 2a, 2b are directly ultrasonic-bonded to each other, there are possibilities that the metal foil, which is thin, is broken, and that the lead 5 is connected at an incorrect position
To address this, the following practice has been conducted: the metal foils of the current collecting tabs 2a, 2b are clamped by clip-shaped plates 4a, 4b, then the clip-shaped plates 4a, 4b are overlapped with the lead 5, and the ultrasonic bonding is performed thereon from above and below. In this case, as the lead 5, a plate member has been used, which has a cross-sectional area uniform over an area thereof from a portion connected to the current collecting tabs 2a, 2b, to the external terminal or to a component part communicating with the external terminal.
Moreover, as the lead 5, a plate member having a rectangular cross section is used in order to facilitate the ultrasonic bonding. The electric resistance of a portion between the electric group and the external terminal needs to be decreased in order to improve the fast charge performance and the high power discharge performance of the battery. The decreasing of the electric resistance then requires an increase in the cross-sectional area of the lead 5.
If the cross-sectional area of the lead 5 is difficult to increase in the direction of the winding axis of the electrode group, that is, in the width direction because of the product limitation, the cross-sectional area is increased in a direction orthogonal to the direction of the winding axis of the electrode group, that is, in the thickness direction.
However, the increase in the thickness of the lead 5 increases its rigidity against an ultrasonic vibration applied to connect the lead 5 to the current collecting tabs 2a, 2b. Accordingly, it becomes difficult for the ultrasonic vibration required for the bonding to be transmitted. As a result, the lead 5 and the metal foils of the current collecting tabs 2a, 2b are not sufficiently bonded, making it difficult to secure a required bond strength.
Alternatively, a laser welding is known as a method of bonding the metal foils of the current collecting tabs 2a, 2b and the lead 5, in addition to the above-described ultrasonic bonding. The laser welding utilizes laser to bond the metal foils of the current collecting tabs 2a, 2b and the lead 5. However, there is a concern that the separator may be damaged by the heat of the laser at a portion bonded by the laser. For this reason, the output of the laser cannot be increased, and therefore, the required bond strength may not be secured.